In 1932 Brillouin discovered that high frequency sound waves can cause diffraction of light. Due to the development of the laser and advances in high frequency acoustic techniques, many applications for this phenomenon have been found such as display devices, laser modulators, tunable filters, acoustic delay lines and acousto-optical waveguide modulators.
A sound wave in a medium produces alternating compression and rarefaction fronts. The index of refraction in these fronts is different, so that the medium acts as a diffraction grating, diffracting light which passes through it, the angle of diffraction increasing as the frequency of the sound waves increases, and the amount of light diffracted increasing with the intensity of the sound wave.
There are two modes of diffraction, the Debye-Sears mode and the Bragg mode. The Debye-Sears mode is obtained if the width of the acoustic beam is less than about .LAMBDA..sup.2 /(4.lambda.) and the Bragg mode is obtained if the width of the acoustic beam is greater than about .LAMBDA..sup.2 /4.lambda. where .LAMBDA. is the acoustic wavelength and .lambda. is the light wavelength. In both modes the acoustic wavelength must be greater than the light wavelength .lambda., and .lambda. must, of course, be within the transparency region of the crystal. In the Debye-Sears mode light enters the crystal parallel to the acoustic wave fronts (0.degree. diffracting angle) and is multiply-diffracted into many images or orders of the initial light beam. In the Bragg mode light enters the crystal at the Bragg Angle .phi. to the acoustic wave fronts where sin .phi. = .lambda./2.zeta.. In this mode the acoustic wavelength and the Bragg angle are matched to the particular light wavelength, and a single order is diffracted from the crystal at the Bragg angle .phi. to the acoustic wave fronts.
A good acousto-optical material should have a high figure of merit M.sub.2, a measure of the amount of light diffracted for a given amount of acoustic power, where M.sub.2 = n.sup.6 p.sup.2 /.rho.v.sup.3 and n is the refractive index, p is the photoelastic coefficient, .rho. is the density, and v is the acoustic velocity. As the formula indicates, a high refractive index and a low velocity will give a high figure of merit. Also, a low velocity will give a greater delay per unit length if the crystal is used in a delay line thus permitting acoustic signal processing devices to have smaller physical dimensions. A good acousto-optical material should also have a low acoustic attenuation, allowing a high frequency wave to propagate a long distance before it is absorbed.
The following table gives a few of the properties of some of the best acousto-optical materials currently known for use in the near infrared region of the spectrum:
Longitudinal Acousto-Optical Material Optical Range Acoustic Velocity Figure of Merit (.mu. m) (.times.10.sup.5 cm/sec) M.sub.2 __________________________________________________________________________ Ge 2-20 5.5 525 As.sub.2 S.sub.3 glass 0.9-11 2.6 230 GaAs 1-11 5.15 93 Tl.sub.3 AsS.sub.4 0.6-12 2.15 330 PbMoO.sub.4 0.4-5.5 3.83 24 __________________________________________________________________________